The host microprocessor of personal computers (PC) has been used to provide the signal processing functions required to implement wire line analog communications functions for some time. The modulator demodulator (MODEM) functions required to implement existing wire line standards were historically implemented in software running on dedicated signal processors on the modem hardware. With the advent of more and more powerful personal computers it became possible to implement much of the processing software on the host processor within the PC. Using the host processor instead of dedicated signal processors greatly reduces the cost of the MODEM hardware. If the host processor is powerful enough, the impact on other application running on the host is relatively small when MODEM processing is performed.
The advent of small low cost radios and small portable notebook computers has spurred the recent growth of wireless local area networks (WLAN). These networks are high bandwidth (>1 Mb/s) half duplex packet networks, which contrast with the wire line networks, which are relatively low bandwidth (<56 kb/s), full duplex, and circuit switched. The same cost savings gained in the wire line network adapters by moving the MODEM functions from dedicated hardware to the PC host processor is theoretically possible with WLAN adapters. However, the bit rates and thus the amount of processing required for WLAN adapters is much higher than that of the wire line adapters. On the surface it would appear that the processing required by the WLAN adapter would make the load on the PC host microprocessor prohibitive.
There are several features of the WLAN waveforms and protocol which can be exploited to allow the host processor in a PC to perform the MODEM functions without significant impact to other applications running on the host processor. First, the WLAN protocol is half duplex, which dictates the host processor is never required to process both transmit and receive waveforms simultaneously. Second, the WLAN protocol is packet based, which means the host processor is not required to process data in a continuous fashion. Third, since the protocol is packet based, every packet is preceded by a preamble. The preambles can be used to trigger the host processor to perform MODEM processing only when valid packets arrive at the receiver.
The WLAN protocol allows mobile stations to notify an access point (AP) that the mobile station (MS) is entering a lower power state. This low power or Power Save (PS) state transmitted by the mobile station is assumed by the AP to prevent packet reception. Therefore, the AP buffers any frames intended for a mobile station in PS state until such time as the mobile station “asks” for individual frames through a polling mechanism or the mobile station returns to the non-PS state. Since a software driver running on the host processor may be adversely affected during times of peak loading, it will benefit from the more regulated behavior of a WLAN mobile station and AP operating in PS mode. While in the PS state, all frames intended for a PS mobile station will be buffered at the AP until the mobile station retrieves them through a PS-Poll message. Normally, this is intended to allow the mobile station to power down the receiver. However, the mechanisms used in this technique (PS mode) will allow a mobile station, whose host processor loading becomes excessive, to defer packet reception until such time as the processor becomes available again.